1. Field of the Invention
This invention relates to a video signal processing apparatus used for a video signal recording/reproducing apparatus such as a video tape recorder and a video disc player for performing line noise cancellation of video signals so as to thereby improve the signal to noise ratio (S/N) of the video signals.
2. Description of the Prior Art
In video signal recording/reproducing apparatus such as video tape recorders (VTR) and video disc players, the luminance signal is recorded on a recording medium with its frequency modulated. The luminance signal, when reproduced from the recording medium, is frequency-demodulated so as to produce the original luminance signal. Since the S/N ratio of the recording medium-reproducing head system is not always sufficient, the S/N ratio of the reproduced luminance signal is also often insufficient, and as a result, noise is perceived on the reproduced picture. For improving the S/N ratio of the luminance signal, the so-called line noise canceller as shown in FIG. 1 has been used. Basically, it eliminates unnecessary spectrum signals by means of a Y-type comb filter so as to improve the S/N ratio, making use of line correlation of the luminance signal. When this is done, a proper measure is provided for preventing the vertical resolution of the picture from being deteriorated. The frequency-modulated luminance signal (FM.sub.Y) is fed to one input terminal of a switch 1. The common terminal of the switch 1 is connected to a demodulator 2 and to a 1H (one horizontal synchronizing period) delay line 3. The output of the 1H delay line 3 is connected to the other input terminal of the switch 1 and to a demodulator 5 through an amplifier 4. The output of the demodulator 5 is applied to a subtractor (or difference calculator) 7 through an amplitude regulator 6. The 7. The output signal of the subtractor 7 is applied to a subtractor (or difference calculator) 10 through an attenuator 8 and a limiter 9. The output of the demodulator 2 is also applied to the subtractor 10. At the output of the subtractor 10 is obtained a luminance signal (Y.sub.out) having an improved S/N ratio.
In FIG. 1, the switch 1, 1H delay line 3 and amplifier 4 constitute a dropout compensator. In a normal state in which no dropout is contained in the luminance signal FM.sub.Y, the switch 1 is positioned as shown in FIG. 1, and the FM.sub.Y is passed to the next stage as is. If a dropout occurs, the switch 1 is shifted to the other state by a signal called D.O produced by a dropout detector (not shown), so that the FM.sub.Y is replaced by the luminance signal inputted 1H before, which contains no dropout. The insertion loss by the 1H delay line is compensated for by the amplifier 4.
The dropout-compensated FM.sub.Y signal is demodulated by the demodulator 2 and applied to the subtractor 7 as the luminance signal of the base band. The FM.sub.Y signal, which passed through the 1H delay line 3 and amplifier 4, is demodulated by the demodulator 5 and turned into the luminance signal of the base band, which is delayed by 1H, and then applied to the subtractor 7 through the amplitude regulator 6. The subtractor 7 forms a C-type comb filter against the demodulated luminance signal. When the output of the subtractor 7 is low (i.e.--the correlation of the luminance signal is high), or within the limited level of the limiter 9, it is applied to the substractor 10 as is. When the output of the subtractor 7 is high (i.e.--the correlation of the luminance signal is low), its amplitude is clipped at a fixed amplitude by the limiter 9 and applied to the subtractor 10. Therefore, the output of the subtractor 10 becomes a signal passed through a Y-type comb filter in an area where correlation of the luminance signal is high, so that the S/N ratio is improved with noise components reduced.
The Y-type comb filter is not formed in an area where correlation of the luminance signal is low, because the amplitude is limited by the limiter 9 and the input signal is outputted as is. Consequently, vertical resolution is not deteriorated. The attenuator 8 is for adjusting the input level of the subtractor 10 so as to be close to the output level of the demodulator 2, because the amplitude of the C-type comb filter (which is the output of the subtractor 7) is double that of the input level.
FIGS. 2(a)-(c) are provided for explaining the operation of the construction of FIG. 1 in terms of its frequency characteristics. FIG. 2(a) shows the frequency characteristic when the correlation of the input luminance signal is low. It shows that the input luminance signal is outputted as is. Although the S/N ratio is not improved, the vertical resolution of the reproduced picture is not deteriorated. FIG. 2(b) shows the frequency characteristic when the correlation of the input luminance signal is high. This is what is called the characteristic of the Y-type comb filter which provides a gain of 1 with respect to a frequency which is an integer multiple higher than the horizontal scanning frequency (f.sub.H) and provides a gain of 0 with respect to frequency (1/2 line offset) which is an odd number of times higher than 1/2 f.sub.H. In this case, noise around the 1/2 line offset frequency are eliminated, so that the S/N ratio is improved. When there is no correlation in the vertical direction of the picture like this, the S/N ratio can be improved without deteriorating the vertical resolution, because the comb filter is formed only in an area where the correlation appears.
However, the above described conventional line noise canceller has some defects. First, since the passband is not of the base band starting from 0 Hz and allows only an RF signal to pass because the 1H delay line is a glass delay line, only a frequency-modulated luminance signal passes. Thus, two or more frequency demodulators are required because the output signal of the delay line and the input signal must be demodulated respectively. Second, since the glass delay line has an insertion loss, the amplifier 4 is Third, since the insertion loss of the glass delay line tends to vary widely, the amplitude regulator 6 is required to form the comb filter. Fourth, the frequency characteristics of the passband of the glass delay fluctuates greatly, and the fluctuation deteriorates the frequency characteristics of the final luminance signal. Due to these reasons, the attenuation rate of the attenuator 8 is normally set low so that the frequency characteristics of the glass delay line will not seriously affect the output. Theoretically, if the attenuation rate is halved, the complete characteristic of the comb filter shown in FIG. 2(b) can be obtained. Actually, however, since the attenuation rate is quartered so that the output will not be affected by the fluctuation of the frequency characteristic of the 1H delay line 3, the comb filter characteristic becomes as shown in FIG. 2(c), and the complete comb filter characteristics cannot be obtained. This not only reduces the improvement of the S/N ratio, but also cannot sufficiently eliminate the cross modulation components of a chroma signal whose spectrum exists at the position of 1/2 line offset.
Furthermore, although not shown in the drawings, since the delay time of the glass delay line also varies widely, the delay time must be adjusted.
As described above, since the conventional line noise canceller uses the glass delay line, various measures are required to cover its characteristics, resulting in a complicated circuit configuration.